1. Field of the Invention
The present invention relates to a lithographic apparatus and device manufacturing method.
2. Description of the Related Art
The term xe2x80x9cpatterning devicexe2x80x9d as here employed should be broadly interpreted as referring to device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term xe2x80x9clight valvexe2x80x9d can also be used in this context. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). An example of such a patterning device is a mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Another example of a patterning device is a programmable mirror array. One example of such an array is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors. In this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronics. In both of the situations described above, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be seen, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT publications WO 98/38597 and WO 98/33096. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required.
Another example of a patterning device is a programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table. However, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (IC""s). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once. Such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus, commonly referred to as a step-and-scan apparatus, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction. Since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be seen, for example, from U.S. Pat. No. 6,046,792.
In a known manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. It is important to ensure that the overlay juxtaposition) of the various stacked layers is as accurate as possible. For this purpose, a small reference mark is provided at one or more positions on the wafer, thus defining the origin of a coordinate system on the wafer. Using optical and electronic devices in combination with the substrate holder positioning device (referred to hereinafter as xe2x80x9calignment systemxe2x80x9d), this mark can then be relocated each time a new layer has to be juxtaposed on an existing layer, and can be used as an alignment reference. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clens.xe2x80x9d However, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a xe2x80x9clensxe2x80x9d. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such xe2x80x9cmultiple stagexe2x80x9d devices the additional tables may be used in parallel or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. Nos. 5,969,441 and 6,262,796.
Referring to FIG. 2, a planar motor of a previously known design for positioning the patterning device and/or the substrate table is shown. Further information on such planar motors may be found in U.S. Pat. No. 6,531,793. The planar motor includes a stationary part 1 and a movable part 2. The stationary part 1 includes a plurality of permanent magnets 3, 4, 5, secured on a carrier. These magnets are arranged in rows 6 and columns 7.
FIG. 3 shows details of the stationary part 1 of the planar motor, including the orientation of the magnets. The arrows indicate the direction of the magnetic polarization of the magnets. In each of the rows 6 and columns 7, adjacent primary magnets 3, 4 are orientated such that their magnetic polarization are anti-parallel to each other and perpendicular to the plane of the stationary part 1 of the planar motor. Between each of the primary magnets 3, 4 additional magnets 5 are arranged in a so-called Halbach configuration, such that their magnetic polarization is perpendicular to that of the primary magnets 3, 4.
FIG. 4 shows details of the moving part 2. For convenience, a set of orthogonal axis X, Y and Z are defined, as shown, such that the X and Y axis are in the plane of the planar motor and the Z axis is perpendicular to the plane of the planar motor. The center of mass 20 of the moving part 2 of the planar motor may be used as the origin. The moving part 2 of the planar motor consists of 8 coil sets 11-18 mounted on a base 19. Each of the coil sets 11-18 are provided with three phase currents which enable the coil sets 11-18, referred to as xe2x80x9cforcers,xe2x80x9d to provide a force in the X or Y direction, depending on their orientation, and a force in the Z direction. The forcers 11-18 are arranged in pairs. As shown in FIG. 4, forcer pairs 11, 12 and 13, 14 are orientated such that their primary axis is in the Y direction. These forcer pairs are therefore able to produce force in the X direction as well as the Z direction. The remaining forcer pairs 15, 16 and 17, 18 are orientated such that their primary axis is in the X direction. These forcer pairs provide forces in the Y direction as well as the Z direction. To produce a force on the moving part 2 in the X direction, the first pair of X-forcers 11, 12 and the second pair of X-forcers 13, 14 operate in unison. When the first pair of X-forcers 11, 12 and the second pair of X-forcers 13, 14 operate in opposite directions a torque about the Z axis is produced on the moving part 2 of the planar motor. Similarly, to produce a force on the moving part 2 of the planar motor in the Y direction, the first pair of Y-forcers 15, 16 and the second pair of Y-forcers 17, 18 operate in unison. When the first pair of Y-forcers 15, 16 operate opposed to the second pair of Y-forcers 17, 18, a torque about the Z axis is also produced. To produce a force in the Z direction, all of the forcers 11-18 produce their Z forces in unison. Torques about the X and Y axis may be produced by operating the first pair of Y-forcers 15, 16 to produce forces in the Z direction in unison with the first pair of X-forcers 11, 12 and opposed to the second pair of Y-forcers 17, 18 and the second pair of X-forcers 13, 14 or by operating the first pair of X-forcers in to produce forces in the Z direction in unison with the second pair of Y-forcers and opposed to the second pair of X-forcers and the first pair of Y-forcers, respectively.
By combining the forces produced in the manner described above, the position and velocity of the moving part 2 of the planar motor may be controlled in all six degrees of freedom (namely in each of the X, Y and Z directions and about axes parallel to each of the X, Y and Z directions). The Z-direction forces of the forcers 11-18 are used to bear the weight of the moving part 2 of the apparatus, thus obviating the requirement for a separate bearing. As shown in FIG. 2, the planar motor operates with the X and Y axis of the moving part 2 at approximately 45xc2x0 to the orientation of the rows 6 and columns 7 of the permanent magnets of the stationary part 1.
However, in addition to the forces in the Z direction and one of the X and Y direction, each of the forcers 11-18 also produces a spurious torque. The X-forcers produce a torque about the Y axis and the Y-forcers produce a torque about the X axis. The torque produced is a function of the force produced by the forcer in the Z direction, the force produced by the forcer in the X or Y direction and the position of a forcer in the X or Y direction. This spurious torque component is referred to as xe2x80x9cpitch torquexe2x80x9d.
The X and Y forcers are provided in pairs in the presently known design of planar motors to counter the effect of the pitch torque. To affect this, each of the forcers in a pair is offset by an amount xcfx84/2. The positional offset is set to be one half of the pitch xcfx84 of the arrangement of the permanent magnets of the stationary part 1 of the planar motor (i.e. half of the distance between the diagonal lines of permanent magnets that are oriented in the same manner, as shown in FIG. 2). Setting the positional offset to this distance xcfx84/2 results in the pitch torque produced by one forcer of a pair precisely offsetting the pitch torque produced by the other forcer of the pair. Therefore the net pitch torque within each pair is zero.
Providing the forcers in pairs is, however, disadvantageous. It doubles the number of forcers required, which increases the mass of the moving part of the planar motor and increases its size in the X-Y plane. This also increases the number of 3-phase amplifiers required (a 90xc2x0 phase shift is required between the 3-phase current system of forcer pairs so each pair of forcers requires two 3-phase amplifiers) as well as the complexity of the control system.
It is an aspect of the present invention to provide a device manufacturing method for use with a lithographic apparatus in which compensation for pitch torque can be effected without the use of additional forcers and amplifiers.
This and other aspects are achieved according to the present invention in a device manufacturing method including providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a beam of radiation using a radiation system; using a patterning device to endow the beam with a pattern in its cross-section; projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material; positioning one of the substrate and the patterning device with a planar motor; the planar motor comprised of a plurality of coil-sets, each producing at least one primary force in a given direction and at least one spurious torque, associated with the primary force, to a moving part of the planar motor, determining the primary forces required to position the planar motor, determining a correction for each of the required primary forces such that the effect of the corrections at least partially compensates for the sum of the spurious torques on the moving part of the planar motor, and providing control signals to the coil-sets to effect the primary forces including the corrections.
This method represents an entirely different mechanism for dealing with pitch torques to the presently known techniques. It is advantageous in that it provides an opportunity to reduce the effect of pitch torque produced by the coil-sets in a planar motor without requiring additional, compensatory, coil-sets on the moving part of the planar motor. It can therefore be tuned by adjusting control parameters rather than by having to make physical alterations to the equipment.
In particular, the correction may be derived from the required primary forces rather than monitoring the difference between the required motion and the observed motion and deriving the correction from this motion. This significantly reduces the overall error in the motion.
The spurious torques that are compensated for in this method are, especially, those which are generated by each coil-set and act about an axis that is perpendicular to the direction of a primary force produced by the coil-set and parallel to the plane of the coils within the coil-set.
Determining the corrections may be performed by calculating the spurious torques that would be caused by the required primary forces being applied by the coil-sets without the corrections and then determining the combination of supplementary primary forces in the coil-sets that produces torques on the moving part of the planar motor that are equal and opposite to the sum of the spurious torques. These supplementary forces are then used as the corrections.
This procedure for compensating for pitch torques significantly reduces the effect of pitch torque using straight-forward control commands.
Alternatively, determining the corrections may be performed by determining correction forces for each of the required primary forces such that, when the sum of the correction forces and the required primary forces are applied by the coil-sets, the sum of the effect of the primary forces and spurious torques substantially equals the forces and torques required to effect the positioning.
This procedure for producing the offset is particularly beneficial since it substantially eliminates the effect of the spurious pitch torques.
According to a further aspect of the invention there is provided a computer program for controlling a lithographic projection apparatus, the computer program comprising codes for instructing the apparatus to: position one of the substrate and the patterning device with a planar motor, the planar motor comprised of a plurality of coil-sets, each producing at least one primary force in a given direction and at least one spurious torque, associated with the primary force, to a moving part of the planar motor; determine the primary forces required to position the planar motor; determine a correction for each of the required primary forces such that the effect of the corrections at least partially compensates for the sum of the spurious torques on the moving part of the planar motor; and provide control signals to the coil-sets to effect the primary forces including the corrections.
According to a further aspect of the invention there is provided a lithographic projection apparatus comprising: a radiation system constructed and arranged to provide a beam of radiation; a support structure constructed and arranged to support a patterning device, the patterning device constructed and arranged to pattern the beam according to a desired pattern; a substrate table constructed and arranged to hold a substrate; a projection system constructed and arranged to project the patterned beam onto a target portion of the substrate; and a planar motor constructed and arranged to position one of the patterning device and the substrate table, the planar motor including a plurality of coil-sets, each producing at least one primary force in a given direction and at least one spurious torque, associated with the primary force, to the a moving part of the planar motor; and a controller that determines the primary forces required to position the planar motor, determines a correction for each of the required primary forces such that the effect of the correction attenuates the sum of the spurious torques on the moving part of the planar motor, and provides a control signal to each of the coil-sets to apply the sum of the required primary forces and the corrections at each coil-set.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget portionxe2x80x9d, respectively.
In the present document, the terms xe2x80x9cradiationxe2x80x9d and xe2x80x9cbeamxe2x80x9d are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.